Paul Boyle was working at Bier Stube on Glasgow's southside when Colin Coats asked him to pass on the message to pub owner Patrick Burns in June 2011, he said.
Coats, 42, and three other men deny the abduction and murder of financial adviser Ms Spence.
The 27-year-old was last seen in Glasgow in April 2011.
At the trial at the High Court in the city, Mr Burns told the jury that in May 2010, Coats had delivered cash to him he understood to be a rent payment from Ms Spence. He said that a year later, Coats demanded the £1200 cash back plus interest.
Giving evidence yesterday, Mr Boyle said Coats asked him to give Mr Burns a message after entering the pub looking angry.
The witness said: "He told me Patrick had made a big mistake, which would mean he'd [Coats] be getting the jail today, which would mean the pub would be going up in flames."
Mr Burns told the jury Ms Spence had owed him rent money, and she had instructed him to meet Coats at the McDonald's car park on Crow Road, Glasgow, where Coats handed him the £1200 cash.
Derek Ogg, defending Coats, suggested to Mr Burns the cash had been a "street loan" from Coats. He said: "You were in desperate need of money, you got a street loan from Mr Coats and you thought you could bump him. You didn't pay him a penny and you eventually had to pay him back."
Mr Burns told the jury that was not correct.
Coats, David Parker, 38, Paul Smith, 47, and Philip Wade, 42, deny torturing Ms Spence for up to a fortnight before killing her in order to obtain financial details.
A former employee of Ms Spence earlier told the court Coats was involved in business dealings with the alleged victim.
Mr Burns, who told the court he had a property firm, said he met Ms Spence in 2009 through her husband, Sokal Zefaj.
He said he was left £20,000 to £30,000 out of pocket by the alleged victim after entering into a property deal.
Mr Ogg suggested Mr Burns had borrowed the £1200 because he was struggling financially.
Mr Burns, 35, said: "I didn't need a loan of money in any desperate way."
His brother, Graham, 28, was general manager of the Bier Stube in Kilmarnock Road when Coats allegedly made his threat.
The court heard he loaned his own BMW Series 3 car to his older sibling because his BMW M6 was off the road.
Graham Burns agreed with a suggestion from Mr Ogg his brother could not afford to get the vehicle back on the road and "he [Patrick Burns] was down on his uppers at that point".
The trial, before Lord Pentland, continues next Thursday.